Ethylene oxide catalyst

ABSTRACT

A silver catalyst for ethylene oxidation to ethylene oxide is provided containing a promoter combination consisting of an alkali metal component, a sulfur component, and a lanthanide component, the catalyst being essentially free of rhenium and transition metal components; optionally the catalyst contains a fluorine component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of silver, alkali metal such as cesium, a lanthanide component and sulfur deposited on a support such as alpha alumina and to the production of ethylene oxide using the catalyst; a fluorine component optionally can be included. The catalyst is essentially free of rhenium or transition metal components.

2. Description of the Prior Art

Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been great efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides a comprehensive analysis of these efforts of prior workers.

Among the many prior teachings in this area is that of U.S. Pat. No. 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).

U.S. Pat. Nos. 5,057,481, and related 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.

U.S. Pat. No. 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.

European Publication 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible copromoters.

U.S. Pat. No. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for purposes of comparison since it is outside the scope of catalyst claimed, the patent shows at columns 21 and 22 a catalyst No. 6 comprised of Ag/Cs/S/F on a support, the Cs amount being 1096 ppm.

U.S. Pat. No. 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.

In the context of the bewildering and vast number of references, many of them contradictory, applicant has discovered a novel and improved catalyst for the production of ethylene oxide.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to an improved supported silver ethylene oxide catalyst containing a promoter combination consisting of a critical amount of an alkali metal component, preferably cesium, together with a sulfur component, and a rare earth or lanthanide component and to the catalyst preparation and use; the catalyst is essentially free of rhenium and transition metal components and optionally can contain a fluorine component.

DETAILED DESCRIPTION

Preferred catalysts prepared in accordance with this invention contain up to about 30% by weight of silver, expressed as metal, deposited upon the surface and throughout the pores of a porous refractory support. Silver contents higher than 20% by weight of total catalyst are effective, but result in catalysts which are unnecessarily expensive. Silver contents, expressed as metal, of about 5-20% based on weight of total catalyst are preferred, while silver contents of 8-15% are especially preferred.

In addition to silver, the catalyst of the invention also contains a critical promoter combination consisting of certain amounts of alkali metal, sulfur and rare earth or lanthanide. The critical amount of alkali metal promoter component is not more than 2000 ppm expressed as alkali metal based on the catalyst weight; preferably the catalyst contains 400-1500 ppm, more preferably 500-1200 ppm alkali metal based on the catalyst weight. Preferably the alkali metal is cesium although lithium, sodium, potassium, rubidium and mixtures can also be used. Impregnation procedures such as are described in U.S. Pat. No. 3,962,136 are advantageously employed for addition of the cesium component to the catalyst.

Necessary also for practice of the invention is the provision of sulfur as a promoting catalyst component. The sulfur component can be added to the catalyst support impregnating solution as sulfate, eg. cesium sulfate, ammonium sulfate, and the like. U.S. Pat. No. 4,766,105 describes the use of sulfur promoting agents, for example at column 10, lines 53-60, and this disclosure is incorporated herein by reference. The use of sulfur (expressed as the element) in amount of 5-300 ppm by weight preferably 50-200 ppm by weight based on the weight of catalyst is essential in accordance with the invention.

The catalyst also contains a rare earth or lanthanide promoter component. As used herein, the terms "lanthanide" or "rare earth" refer to the rare earth metals or elements having atomic numbers 57 through 71 in the Periodic Table of the Elements i.e., lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Preferred are praseodymium, neodymium, europium, terbium, dysprosium and holmium. Holmium is especially preferred. Mixtures of the rare earth components can be used. Promethium, being radioactive, is not preferred.

In the catalysts of the invention the rare earth or lanthanide component, expressed as the element, is used in amount of 10-500 ppm, preferably 50-300 ppm by weight based on the weight of the catalyst. The lanthanide component can be added to the catalyst support impregnating solution as a sulfate, chloride, acetate, nitrate or other salt.

The catalyst also optionally may contain a fluorine promoter in amount expressed as the element F of 10-300 ppm, preferably 50-200 ppm by weight based on the catalyst as an optional component. Ammonium fluoride, alkali metal fluoride, and the like can be used.

The catalysts are made with supports comprising alumina, silica, silica-alumina or combinations thereof. Preferred supports are those containing principally alpha-alumina, particularly those containing up to about 15 wt % silica. Especially preferred supports have a porosity of about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferred supports also have a relatively low surface area, i.e. about 0.2-2.0 m² /g, preferably 0.4-1.6 m² /g and most preferably 0.5-1.3 m² /g as determined by the BET method. See J. Am. Chem. Soc. 60, 3098-16 (1938). Porosities are determined by the mercury porosimeter method; see Drake and Ritter, "Ind. Eng. Chem. anal. Ed.," 17, 787 (1945). Pore and pore diameter distributions are determined from the surface area and apparent porosity measurements.

For use in commercial ethylene oxide production applications, the supports are desirably formed into regularly shaped pellets, spheres, rings, etc. Desirably, the support particles may have "equivalent diameters" in the range from 3-10 mm and preferably in the range of 4-8 mm, which are usually compatible with the internal diameter of the tubes in which the catalyst is placed. "Equivalent diameter" is the diameter of a sphere having the same external surface (i.e. neglecting surface within the pores of the particle) to volume ratio as the support particles being employed.

Preferably, the silver is added to the support by immersion of the support into a silver/amine impregnating solution or by the incipient wetness technique. The silver containing liquid penetrates by absorption, capillary action and/or vacuum into the pores of the support. A single impregnation or a series of impregnations, with or without intermediate drying, may be used, depending in part upon the concentration of the silver salt in the solution. To obtain catalyst having silver contents within the preferred range, suitable impregnating solutions will generally contain from 5-50 wt % silver, expressed as metal. The exact concentrations employed, of course, will depend upon, among other factors, the desired silver content, the nature of the support, the viscosity of the liquid, and solubility of the silver compound.

Impregnation of the selected support is achieved in a conventional manner. The support material is placed in the silver solution until all of the solution is absorbed by the support. Preferably the quantity of the silver solution used to impregnate the porous support is no more than is necessary to fill the pore volume of the porous support.

The impregnating solution, as already indicated, is characterized as a silver/amine solution, preferably such as is fully described in U.S. Pat. No. 3,702,259 the disclosure of which is incorporated herein by reference. The impregnation procedures described in U.S. Pat. No. 3,962,136 are advantageously employed for the cesium component.

Known prior procedures of predeposition, co-deposition and postdeposition of the various promoters can be employed.

After impregnation, any excess impregnating solution is separated and the support impregnated with silver and the promoter or promoters is calcined or activated. In the most preferred practice of the invention, calcination is carried out as described in commonly assigned U.S. Pat. No. 5,504,052 and U.S. Pat. No. 5,646,087, the disclosures of which are incorporated herein by reference. The calcination is accomplished by heating the impregnated support, preferably at a gradual rate, to a temperature in the range 200-500° C. for a time sufficient to convert the contained silver to silver metal and to decompose the organic materials and remove the same as volatiles.

The impregnated support is maintained under an inert atmosphere while it is above 300° C. during the entire procedure. While not wishing to be bound by theory, it is believed that at temperatures of 300° C. and higher oxygen is absorbed in substantial quantities into the bulk of the silver where it has an adverse effect on the catalyst characteristics. Inert atmospheres as employed in the invention are those which are essentially free of oxygen.

An alternative method of calcination is to heat the catalyst in a stream of air at a temperature not exceeding 300° C., preferably not exceeding 250° C.

Catalysts prepared in accordance with the invention have improved performance, especially stability, for the production of ethylene oxide by the vapor phase oxidation of ethylene with molecular oxygen. These usually involve reaction temperatures of about 150° C. to 400° C., usually about 200° C. to 300° C., and reaction pressures in the range of from 0.5 to 35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15% oxygen, with the balance comprising comparatively inert materials including such substances as nitrogen, carbon dioxide, methane, ethane, argon and the like. Only a portion of the ethylene usually is reacted per pass over the catalyst and after separation of the desired ethylene oxide product and the removal of appropriate purge streams and carbon dioxide to prevent uncontrolled build up of inerts and/or by-products, unreacted materials are returned to the oxidation reactor.

A disadvantage of the prior art rhenium promoted catalysts has been the instability associated with such catalysts. In accordance with the present invention, the rhenium free catalysts have advantageously high selectivity and high stability.

The following examples illustrate the invention.

EXAMPLE 1

A silver solution was prepared using the following components (parts are by weight):

Silver oxide--834 parts

Oxalic acid--442 parts

Deionized water--2808 parts

Ethylene Diamine--415 parts

Silver oxide was mixed with water, at room temperature, followed by the gradual addition of the oxalic acid. The mixture was stirred for 15 minutes and at that point the color of the black suspension of silver oxide had changed to the gray/brown color of silver oxalate. The mixture was filtered and the solids were washed with 3 liters of deionized water.

A container which contained the washed solids was placed in an ice bath and stirred while ethylene diamine and water (as a 72%/28% mixture) were added slowly in order to maintain the reaction temperature lower than 33° C. After the addition of all the ethylene diamine water mixture the solution was filtered at room temperature. The clear filtrate was utilized as a silver/amine stock solution for the catalyst preparation.

The support used for the examples was obtained from Norton Company and was made primarily of alpha-alumina in the form of 5/16 inch cylinders. The support has a surface area of 0.65 m² /g, pore volume of 0.3 cc/g, and median pore diameter of 1.5μ. For the examples, about 185 parts of the silver solution were mixed with varying amounts of:

1. CsOH solution, (8% Cs by weight in water),

2. ammonium fluoride, (3% F by weight in water)

3. ammonium hydrogen sulphate, (1% S by weight in water) and aqueous solution of the indicated lanthanide compound, the amounts of the promoter solutions being adjusted to give the promoter concentrations indicated in the tables. In certain comparative runs the use of various promoter solutions was omitted to give the indicated compositions. See Runs 1*, 2*, 3*, 4*, 6*, 7*, 8*, 9*, 10*, 11*, 12*, 13*, 14* and 15* in Table 2.

The mixture of silver stock solution and promoter solutions was stirred to assure homogeneity, then added to 400 parts of the support. The wet catalyst was mixed for ten minutes and then calcined.

Calcination, the deposition of silver compound, was induced by heating the catalyst up to the decomposition temperature of the silver salt. This was achieved via heating in a furnace that has several heating zones in a controlled atmosphere. The catalyst was loaded on a moving belt that entered the furnace at ambient temperature. The temperature was gradually increased as the catalyst passed from one zone to the next. It was increased, up to 400° C., as the catalyst passed through seven heating zones. After the heating zones the belt passed through a cooling zone that gradually cooled the catalyst to a temperature lower than 100° C. The total residence time in the furnace was 22 minutes. Atmosphere of the furnace was controlled through use of nitrogen flow in the different heating zones. In some instances, as indicated in the following table the calcination was carried out with air.

The catalysts were tested in a tube which was heated by a salt bath. A gas mixture containing 15% ethylene, 7% oxygen, and 78% inert, mainly nitrogen and carbon dioxide, was passed through the catalyst at 300 p.s.i.g., the temperature of the reaction was adjusted in order to obtain ethylene oxide productivity of 160 Kg per hour per m³ of catalyst and this temperature is given in the Table.

The results of the catalyst tests are summarized in Table 1.

                                      TABLE 1     __________________________________________________________________________                     Lanthanide         Cs  F   S   metal       Temp metal     Run ppm ppm ppm ppm    Sel %                                 ° C.                                      cpd.     __________________________________________________________________________     1   565 75  85  150 La 83.3 236  La.sub.2 (SO.sub.4).sub.3     2   590 75  34  150 Ce 83.0 232  CeCl.sub.3     3   568 75  102 150 Ce 83.3 238  Ce(SO.sub.4).sub.2     4   612 75  34  155 Pr 83.7 240  PrCl.sub.3     5   607 75  34  160 Nd 83.7 240  NdCl.sub.3     6   586 75  51  165 Sm 83.2 238  Sm.sub.2 (SO.sub.4).sub.3     7   888 75  52  165 Eu 84.1 244  Eu.sub.2 (SO.sub.4).sub.3     8   582 75  34  165 Eu 83.9 238  EuCl.sub.3     9   592 75  34  170 Gd 83.3 239  Gd(NO.sub.3).sub.3     10  601 75  53  175 Tb 83.6 238  Tb.sub.2 (SO.sub.4).sub.3     11  616 75  34  175 Tb 83.6 242  Tb(NO.sub.3).sub.3     12  598 75  34  175 Tb 83.8 237  TbCl.sub.3     13  638 75  52  180 Dy 83.7 236  Dy.sub.2 (SO.sub.4).sub.3     14  598 75  34  180 Dy 83.9 240  DyCl.sub.3     15  633 75  34  180 Ho 84.4 240  HoCl.sub.3     16  628 75  34  180 Ho 83.8 240  Ho(NO.sub.3).sub.3     17  597 75  53  185 Er 83.5 237  Er.sub.2 (SO.sub.4).sub.3     18  621 75  34  185 Er 83.5 238  Er(NO.sub.3).sub.3     19  611 75  34  185 Er 83.6 237  ErCl.sub.3     20  631 75  53  185 Tm 83.4 236  Tm.sub.2 (SO.sub.4).sub.3     21  637 75  53  186 Yb 83.6 235  Yb.sub.2 (SO.sub.4).sub.3     22  618 75  34  189 Lu 83.5 237  Lu(NO.sub.3).sub.3     __________________________________________________________________________

From the results given above, it can be seen that the use of the lanthanide promoters in combination with cesium and sulfur components provides significantly improved catalyst performance.

The effect of fluoride can be seen in the results shown below in Table 2. The addition of 75 ppm F can, in some cases, result in a dramatic, cooperative increase in selectivity over that of the non-fluoride promoted catalysts. In the table shown below, the best cases for non-fluoride and fluoride promoted catalysts are presented. In each instance, the lanthanide promoted catalysts outperformed the 82.8% selectivity achieved with similar levels of cesium, fluorine, and sulfur alone.

                  TABLE 2     ______________________________________                                 Lanthanide           CS      F       S     metal     Run   ppm     ppm     ppm   ppm     Sel % Temp ° C.     ______________________________________     *1    558     0       34    150 La  82.3  240     1     565     75      34    150 La  83.3  236     *2    607     0       34    150 Ce  81.9  234     3     568     75      102   150 Ce  83.3  238     *3    562     0       34    155 Pr  83.1  232     4     612     75      34    155 Pr  83.7  240     *4    578     0       34    160 Nd  82.9  234     5     607     75      34    160 Nd  83.7  240     *5    501     0       52    165 Eu  82.4  236     7     888     75      52    165 Eu  84.1  244     *6    465     0       52    170 Gd  82.7  231     9     611     75      52    170 Gd  83.3  242     *7    536     0       53    175 Tb  82.5  233     12    598     75      34    175 Tb  83.8  238     *8    455     0       53    180 Dy  81.8  226     14    598     75      34    180 Dy  83.9  236     *9    500     0       34    180 Ho  82.2  231     15    633     75      34    180 Ho  84.4  240     *10   493     0       53    185 Er  82.5  230     18    611     75      34    185 Er  83.6  237     *11   606     0       53    185 Tm  83.3  235     20    631     75      53    185 Tm  83.4  236     *12   625     0       53    186 Yb  83.1  235     21    637     75      53    186 Yb  83.6  235     *13   612     0       53    189 Lu  83.2  236     22    618     75      34    189 Lu  83.5  237     *14   611     0       34    0       82.4  237     *15   606     80      34    0       82.8  238     ______________________________________      *Comparative 

I claim:
 1. A rhenium and transition metal free catalyst for the oxidation of ethylene to ethylene oxide consisting essentially of silver on a solid support and containing a promoter combination consisting essentially of (1) a cesium component in amount not greater than 1500 ppm based on the weight of the catalyst, (2) a sulfur component in amount of 5-300 ppm based on the weight of the catalyst, and (3) a lanthanide component in amount of 10-500 ppm based on the weight of the catalyst.
 2. The catalyst of claim 1 wherein the cesium component is in amount of 400-1000 ppm based on the weight of the catalyst.
 3. The catalyst of claim 1 wherein the support is alpha alumina.
 4. The catalyst of claim 1 comprised by weight of 5-20% silver.
 5. The catalyst of claim 1 additionally containing 10-300 ppm of a fluorine component.
 6. The catalyst of claim 1 wherein the lanthanide is praseodymium in the amount of 10-500 ppm based on the weight of the catalyst.
 7. The catalyst of claim 1 wherein the lanthanide is neodymium in the amount of 10-500 ppm based on the weight of the catalyst.
 8. The catalyst of claim 1 wherein the lanthanide is europium in the amount of 10-500 ppm based on the weight of the catalyst.
 9. The catalyst of claim 1 wherein the lanthanide is terbium in the amount of 10-500 ppm based on the weight of the catalyst.
 10. The catalyst of claim 1 wherein the lanthanide is dysprosium in the amount of 10-500 ppm based on the weight of the catalyst.
 11. The catalyst of claim 1 wherein the lanthanide is holmium in the amount of 10-500 ppm based on the weight of the catalyst. 